A U.S. human rights group focused on Iran says a relative of a detainee at a women's prison near Tehran says there was a violent security crackdown on a protest by female inmates.

In a report published Friday, the Washington-based Abdorrahman Boroumand Center (ABC) quoted the family member of the detainee at Qarchak prison as saying the incident happened the day before.

The ABC source said female inmates at the prison, also known as Shahr-e-Rey, in the town of Varamin staged a protest on Thursday after learning that they would not be freed as part of an amnesty to mark the 40th anniversary of Iran's Islamic Revolution on Feb. 11.

Pardons promised

ABC quoted the source as saying prison officials recently promised the one thousand female inmates at Qarchak that Iran's Supreme Leader would pardon most of them, besides those convicted of murder, as part of the amnesty plan. But ABC cited the source as saying prison officials released a smaller-than-expected list of pardoned women on Thursday, prompting angered inmates to pound on their doors and walls.

The source told ABC that prison guards responded by hitting the protesters with batons and firing tear gas in windowless prison halls, causing some inmates to have problems breathing.

There was no mention of the incident by Iranian state media.

London-based rights group Amnesty International said it had seen several reports of Iranian guards beating prisoners and using tear gas at Qarchak prison.

'Appalling ill-treatment'

In a Friday statement, Amnesty's Middle East research director Philip Luther said he was "deeply alarmed" by the reports of what he called a "reckless and heavy-handed" response by Iranian prison guards to the protest. He said many prisoners were reported to have received hospital treatment for the effects of tear gas.

"Prison authorities must refrain from using unnecessary and excessive force against prisoners," Luther said. "Instead of carrying out violent raids against prisoners, they should be working to address the inhumane and squalid conditions at Shahr-e Rey prison."

Amnesty said it has previously documented cases of "appalling ill-treatment" of prisoners at Qarchak. It said inmates at the site, which used to be a chicken farm, are held in overcrowded and unhygienic conditions, without access to safe drinkable water, decent food, medicine and fresh air.